This invention relates to a ship intended for traffic in icy waters and to a hull for such a ship.
An important parameter in the design of a ship intended for traffic in ice-filled waters is the maximum thickness of level ice to be broken by the ship. This parameter affects at least the following design decisions, namely, hull plating thickness, particularly for the bow portion of the hull (the bow plating is typically 30 mm for ice 1 m thick, 40 mm for ice 1.5 m thick and 50 mm for ice thicker than 1.5 m); bow form (a rounded-off form is preferable for thicker ice); stem angle (the thicker the ice, the greater the inclination of the stem); and rudder position (the thicker the ice, the greater the depth of the rudder below the design waterline plane).
As a ship moves through an ice field, the ice is broken into chunks and chunks in front of the ship are forced below the surface of the water by the ship's forward movement. The chunks of ice then slide along the external surface of the underwater part of the hull. Some of the chunks of ice slide along the bottom surface of the hull, thereby easily coming in contact with the ship's propeller, which reduces propeller efficiency.
A considerable portion of the resistance to movement encountered by an ice breaking ship in icy waters is due to friction between the ship's hull and the chunks of broken ice sliding along the underwater part of the hull. This friction may be decreased by directing ice chunks so that they move away from the traffic channel formed by breaking the ice and pass under the adjacent edges of the unbroken ice. At the same time, the advantage is obtained that the ice chunks are kept away from the propellers of the ship, which improves the efficiency of the propellers. In addition, the passage of ships following the ice breaking ship is facilitated.
The size of the ice chunks formed when a ship traverses an ice field depends largely on the bow form. The size of a chunk refers to the average linear dimension of the chunk in the plane of the ice. The dimension is usually fairly uniform in all horizontal directions, since ice chunks seldom have a distinctly elongated form. It will be appreciated that the total energy used in breaking ice is in most cases inversely related to the size of the chunks. One explanation for this is that formation of a few large chunks implies that the total length of fractures in the ice is smaller than if many small chunks are formed. Therefore, the size of the ice chunks is an important parameter in the design of a ship for use in icy water. In terms of ice-breaking efficiency, therefore, it is desirable to design a ship to form large chunks by bending the ice downwards. However, a bow form that is favorable for ice breaking is generally unfavorable with respect to its resistance to forward movement in open water, and, therefore, a compromise must be made based on the proportion of time that the ship is expected to be operating in icy water.
Several different ship hull forms have been suggested for directing ice chunks away from under the bottom of a ship. U.S. Pat. No. 4,715,305 discloses a vessel whose bottom includes a step-formed plow structure diverging in a rearward direction. Disadvantages of a hull provided with such a plow structure are that it is expensive to build and, in comparison with a conventional ship hull, is less seaworthy and has a greater resistance to movement in open water. The plow structure also increases the draft of the vessel and reduces its displacement in relation to its draft.
Another way of directing ice chunks away from under the bottom of a ship is shown in German Application Publication DE 2112334, according to which vertically projecting plow elements are fitted to the bottom of the ship. This structure gives a relatively good sideways transport of ice chunks, but it considerably increases the vessel's draft and resistance to forward movement. It also causes problems when the ship is to be docked. In addition, a relatively strong turbulent water flow occurs under the lower edge of the plow and this tends to suck ice chunks in under the bottom of the ship and defeat the purpose of the plow.
U.S. Pat. No. 4,702,187 shows a plow structure that is foldable into a recess in the bottom of the ship. When the plow is in its folded position, the resistance to forward movement in free water is smaller, the ship is more seaworthy, its steering is more easy and the draft of the ship is not greater than that of a ship without a plow structure. Further, when the apex of the plow is open or the rear side of the plow is inclined towards the bottom of the ship, the undesirable turbulent flow under the bottom edge of the plow is decreased. Disadvantages of the foldable plow include the practical problems related to moving structures submerged in water, and strength problems, because a large load is applied to the plow by the ice chunks.